This is a very good response, that allows us to make our disagreement more precise. I agree that choosing menstruation, or its hypothetical unisex counterpart, is unreasonable because it is too early. I disagree that birth is too early in the same way. Pretty much everyone in our society agrees that 12-year olds cannot meaningfully consent to sex (especially with adults), whereas many believe 6-month old children to be people -- in fact, many believe fetuses to be people! You might say that they are obviously wrong, but the "obviously" is suspicious when so many disagree with you, at the very least for Aumann reasons.

To put it in another way: What makes you so certain that birth is so far off from what is reasonable as a line for personhood, when you are willing to draw your line at 10 months? That is much closer to birth than 17 is to 12 years old.

Also, I think your analogy needs a bit of amending: the relevant question is, if there was a visible unisex menstruation happening at 17 years old, and an established tradition of taking that as the age of consent, why on earth would a society change the law to make it 16 years and 2 months instead?